1. Field of the Invention
The present invention relates to a glass paste. More particularly, the present invention also relates to a glass paste used for a partition wall and the like formed on a substrate of a plasma display panel (hereinafter, referred to as xe2x80x9cPDPxe2x80x9d).
2. Description of the Related Art
In CRT and color liquid crystal displays, increase in size of screen is difficult, on the other hand, PDP is a flat panel display enabling it, and expected to be used for indicator or as a large screen television in public spaces. PDP has a mechanism in which electric discharge is caused between electrodes in a space partitioned with partition walls and filled with a rare gas, and ultraviolet ray emitted from the generated plasma is converted by a phosphor into visible light, to cause showing images. The partition wall is made of glass, and produced by molding a glass powder and calcining the molded article for densification. Addition of inorganic powders such as alumina, zircon and the like into a glass powder has been conventionally suggested. An object of the addition of these inorganic powders is to keep the molded form of the glass in melt condition in the process of calcination after molding of a partition wall using a glass powder.
If the reflective index of a partition wall molded from a glass powder can be improved, light emitted from a phosphor applied on the surface of the partition wall can be efficiently used for display, and the brightness of a screen can be substantially improved. Conventionally, alumina and zircon are used as inorganic fillers, however, they have insufficient function to reflect light. There is a possibility of improvement in brightness, by using an inorganic powder as a filler in a partition wall glass having higher refractive index than alumina (refractive index: 1.77) and zircon (refractive index: 1.9) and by reflecting at the partition wall, light emitted from a phosphor toward rear side of a panel, toward front side of the panel. Regarding an idea of using titanium oxide having a refractive index of 2.6 as a reflection material, JP-A No. 8-321257, for example, has a disclosure: xe2x80x9cFor the purpose of introducing light emission of a phosphor effectively to the front side of a panel, it may be more advantageous in a certain case to render the color of a partition wall white. In this case, titania and the like are used as a refractory white pigmentxe2x80x9d. However, titanium oxide (titania) tends to weaken reflection of light from a blue light-emitting phosphor, and is not sufficient as a filler having high reflection function.
Further, JP-A No. 11-60273 discloses an example in which a glass film having a film thickness of 15 xcexcm obtained by adding a titanium oxide powder to a glass powder and calcining the mixture has reflective index of 70%, 65% and 62%, respectively, at wavelengths of 460 nm, 550 nm and 620 nm. However, in this example, titanium oxide is used as an inorganic filler, and a problem of insufficient reflection of light emitted from a blue light-emitting phosphor is not solved.
For enhancing an ability to reflect light, high refractive index is necessary. A partition wall in PDP is constituted of glass, and if a difference in refractive index is small between the glass and inorganic powder as a filler, an ability to reflect light at the interface of the filler particle in glass matrix decreases, therefore, the refractive index is preferably 2.0 or more which is significantly larger than that of the glass matrix. Though there is no upper limit on refractive index, a refractive index of 2.6 of titania is the highest, among substances which can be usually used industrially. However, in spite of high refractive index, titania shows insufficient reflection of light emitted from a blue light-emitting phosphor. For an inorganic powder suitable as a filler for a glass paste which will become a PDP partition wall, only high value of refractive index is not sufficient, and it is also required that reflective index of light measured in the form of a powder is high, and reflective index is high at red, blue and green lights having specific wavelengths, namely, at from 400 nm which is a wavelength near the lower limit on shorter wavelength side of visible light to 700 nm which is a wavelength near the upper limit on longer wavelength side of visible light.
An object of the present invention is to provide a glass paste having high function to reflect light, obtained by adding an inorganic powder to a glass material, which is used for a partition wall and the like formed on a substrate of PDP.
The present inventors have found that a glass paste comprising an inorganic powder having a refractive index of not less than a specific value, having high reflective index at red, blue and green light having specific wavelengths, and having a particle size within a specific range, is suitable for a PDP partition wall.
Namely, the present invention provides a glass paste comprising an inorganic powder, wherein the powder has a refractive index of 2.0 or more, reflective index at wavelengths of light of 400 nm, 550 nm and 700 nm in a light reflection spectrum of 80% or more, the primary particle size measured by scanning electron microscopy (SEM) of from 0.1 xcexcm to 10 xcexcm, and the BET specific surface area of from 0.1 m2/g to 15 m2/g.